As a power source for driving a cordless electric tool, used is a secondary cell having a capacity relatively increased such as a nickel hydrogen battery or a NiCd battery. As the secondary cell having a capacity more increased and a weight more decreased, a lithium cell including a lithium ion cell has been progressively practically used.
The lithium battery has a performance that the nominal voltage of the cell is about three times as high as that of the nickel hydrogen battery or the NiCd battery widely put to practical use, and the energy density thereof is about three times as high as the NiCd battery and a feature that the battery is compact and light. Further, the lithium battery has features that the battery has good discharge efficiency, the battery can be discharged even in an environment of relatively low temperature and the battery can obtain a stable voltage in a wide range of temperature.
The lithium battery (battery pack) is ordinarily charged under a constant current/constant voltage control as shown in a charging characteristic view of FIG. 4. However, it is generally known that to overcharge the battery under the constant current/constant voltage control is undesirable in view of the characteristics or the life of the battery. Accordingly, for instance, as disclosed in JP-A-2-192670, a method is proposed that while a battery is charged under a constant voltage, the charging current of a charging power circuit is detected, and when the charging current is lowered to a value not higher than a prescribed value, the battery is decided to be fully charged to generate a charging stop signal, interrupt a charging path of the charging power circuit and finish a charging operation.